In the field of computer systems, significant advances have been made in recent years in providing affordable mass storage with increased storage capacity and decreased access time. Much of this effort has been directed at rotating magnetic media, such as that found in hard disk drives. Unfortunately, access times with magnetic media remain long, in the millisecond range.
In an effort to decrease access times and increase storage capacity, holographic storage systems have been developed. Examples of such holographic storage systems are described in U.S. Pat. No. 4,927,220, entitled "SYSTEM AND METHOD FOR PHOTOREFRACTIVE HOLOGRAPHIC RECORDING AND SIGNAL PROCESSING," issued on May 22, 1990. That reference is herein incorporated by reference.
Holographic storage systems offer significant advantages over conventional mass storage systems. For example, the access time with holographic storage systems is on the order of microseconds. Furthermore, holographic storage systems retrieve arrays of data bits in parallel, rather than serially. For example, an array of 1,000 bits by 1,000 bits can be retrieved at once.
Holographic storage capacity is increased by storing multiple holograms at the same location on the recording medium. This storage of multiple holograms at the same location, referred to as multiplexing, can be accomplished in at least two ways: angle encoding and phase encoding.
With angle encoding, different holograms can be stored at the same location by changing the angle of the reference beam used to record each hologram. To prevent cross-correlation between holograms stored at the same location, each reference beam should be separated by the Bragg selectivity angle of the recording medium. The Bragg selectivity angle of the recording medium is a function of the thickness of the recording medium. In particular, the Bragg selectivity angle decreases as the thickness of the storage medium increases. Thus, systems with smaller Bragg selectivity angles allow for greater hologram multiplexing.
With phase encoding, reference beams are encoded with particular phase patterns. Recall of a particular hologram is accomplished by applying the same phase code to the reference beam as was used to record the hologram. At least two types of phase encoding have been successfully applied to holographic storage systems. Unfortunately, both have drawbacks.
The first type of phase encoding is referred to as orthogonal phase encoding, and uses orthogonal codes, for example, Walsh codes. With orthogonal coding, the reference beam is angularly divided into n segments. For each stored hologram, each segment is phase-shifted either 0 or .pi. radians, so that the reference beam associated with each hologram is orthogonal to all others. On recall, this phase-shifting results in destructive reconstruction of each recorded hologram, except for the one recorded with the correspondingly encoded reference beam.
With orthogonal phase encoding, each segment of the reference beam should be separated by the Bragg selectivity angle, so that recall is a function only of the inner product of the reference beam segments. Otherwise, cross-talk arises during recall. Thus, the usefulness of orthogonal encoding depends on the Bragg selectivity angle of the system. For example, with a Bragg selectivity angle of 3 degrees, and an optical system allowing 24 degrees of reference beam freedom, a reference beam can be divided into only 8 segments, and thus a maximum of 8 holograms can be stored at one location in the medium.
With the second phase encoding technique, referred to as random phase encoding, the reference beam is encoded with random phase patterns for recording and recall of holograms. With random phase encoding, the reference beam is not divided into different segments, and there is no Bragg selectivity angle restriction. However, cross-correlation between random patterns results in recalls that may have high cross correlation. Therefore, the number of holograms that can be stored at a particular location using random phase encoding is limited by the maximum cross-correlation noise that can be tolerated.
Because of these limitations, a need has arisen for a method and apparatus for phase encoding of reference beams that allows for the storage of a greater number of holograms with acceptable cross correlation noise.